Operating a trunked communication system in transmission and message trunked modes

ABSTRACT

A digitally trunked radio repeater system provides substantial improvements in timeliness and reliability. The mobile radio transceivers transmit channel requests on the control channel at 9600 bps. The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. Radios select between message and transmission trunking based on over-the-air signaling. During transmission trunking, called and calling receivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation.

This is a divisional of application Ser. No. 08/425,152 filed Apr. 19,1995, now U.S. Pat. No. 5,574,788, which in turn is a divisional of Ser.No. 08/105,153, filed Aug. 12, 1993, now U.S. Pat. No. 5,483,670 issuedJan. 9, 1996, which in turn is a divisional of Ser. No. 07/860,159 filedMar. 30, 1992, now U.S. Pat. No. 5,274,837 issued Dec. 28, 1993, whichin turn is a divisional of Ser. No. 07/464,053, filed Jan. 3, 1990, nowU.S. Pat. No. 5,125,102, issued Jun. 23, 1992, which in turn is adivisional of application Ser. No. 07/056,922, filed Jun. 3, 1987, nowU.S. Pat. No. 4,905,302, issued Feb. 27, 1990.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention is generally directed to the art of trunked radiorepeater systems. It is more particularly directed to such systems usingdigital control signals transmitted over a dedicated control channelwhile also using plural working channels which are assigned temporarilyfor use by individual radio units.

The trunking of radio repeaters is well-known. Early trunk systems usedanalog control signals while some more recent systems hase utilizeddigital control signals. Control signals haze been utilized on adedicated control channel and/or on different ones of the workingcnannels for various different reasons and effects. A non-exhaustive butsomewhat representative sampling of prior art publications and patentsdescribing typical prior art trunked radio repeater systems isidentified below:

U.S. Pat No. 3,292,178--Magnuski (1966)

U.S. Pat No. 3,458,664--Adlhoch et al (1969)

U.S. Pat No. 3,571,519--Tsimnbidis (1971)

U.S. Pat No. 3,696,210--Peterson et al (1972)

U.S. Pat No. 3,906,166--Cooper et al (1975)

U.S. Pat No. 3,936,616--DiGianfilippo (1976)

U.S. Pat No. 3,970,801--Ross et al (1976)

U.S. Pat No. 4,001,693--Stackhouse etal (1977)

U.S. Pat No. 4,010,327--Kobrinetz et al (1977)

U.S. Pat No. 4,012,597--Lynk, Jr. et al (1977)

U.S. Pat No. 4,022,973--Stackhouse etal (1977)

U.S. Pat No. 4,027,243--Stackhouse etal (1977)

U.S. Pat No. 4,029,901--Campbell (1977)

U.S. Pat No. 4,128,740--Graziano (1978)

U.S. Pat No. 4,131,849--Freeburg et al (1978)

U.S. Pat No. 4,184,118--Cannalte et al (1980)

U.S. Pat No. 4,231,114--Dolikian (1980)

U.S. Pat No. 4,309,772--Kloker et al (1982)

U.S. Pat No. 4,312,070--Coombes et al (1982)

U.S. Pat No. 4,312,074--Pautler et al (1982)

U.S. Pat No. 4,326,264--Cohen et al (1982)

U.S. Pat No. 4,339,823--Predina et al (1982)

U.S. Pat No. 4,347,625--Williams (1982)

U.S. Pat No. 4,360,927--Bowen et al (1982)

U.S. Pat No. 4,400,585--Kamen et al (1982)

U.S. Pat No. 4,409,687--Berti et al (1983)

U.S. Pat No. 4,430,742--Milleker et al (1984)

U.S. Pat No. 4,430,755--Nadir et al (1984)

U.S. Pat No. 4,433,256--Dolikian (1984)

U.S. Pat No. 4,450,573--Noble (1984)

U.S. Pat No. 4,485,486--Webb et al (1984)

U.S. Pat No. 4,578,815--Persinotti (1985)

Bowen et al is one example of prior art switched channel repeatersystems which avoid using a dedicated control channel--in part byproviding a handshake with the repeater site controller on a seized"idle" working channel before communication with the called unit(s) ispermitted to proceed.

There are many actual and potential applications for trunked radiorepeater systems. However, one of the more important applications is forpublic service trunked (PST) systems. For example, one metropolitan areamay advantageously utilize a single system of trunked radio repeaters toprovide efficient radio communications between individual radio unitswithin many different agencies. Each agency may, in turn, achieveefficient communication between individual units of different fleets orsub-units (e.g., the police department may have a need to provideefficient communications between different units of its squad car force,different portable units assigned to foot patrolmen, different units ofdetectives or narcotics agents and the like). Sometimes it may beimportant to communicate simultaneously to predefined groups of units(e.g., all units, all the squad cars, all of the foot patrolmen, etc.).At the same time, other agencies (e.g., the fire department, thetransportation department, the water department, the emergency/rescuese-vices, etc.) may be in need of similar communication services. As iswell-known to those familiar with trunking theory, a relatively smallnumber of radio repeaters can efficiently service all of these needswithin a given geographic area if they are trunked (i.e., shared on an"as-needed" basis between all potential units).

This invention also is especially adapted for special mobile radio (SMR)trunked users. Here, an entrepreneur may provide a trunked radiorepeater system at one or more sites within a given geographic area andthen sell air time to many different independent businesses or otherentities having the need to provide efficient radio communicationbetween individual units of their particular organization. In manyrespects, the requirements of an SMR user are similar to those of a PSTuser.

In fact, the potential advantages of trunked radio repeater systems forpublic services is so well recognized that an organization known as theAssociation of Public-Safety Communications Officers, Inc. (formerly theAssociation of Police Communications Officers) (APCO) has developed aset of highly desirable features for such a system commonly known as the"APCO-16 Requirements." A complete listing and explanation of suchrequirements may be found in available publications known to those inthe art.

One of the APCO-16 requirements is that any user must have voice channelaccess within one-half second after engaging a push-to-talk (PTT)switch. This same requirement must especially be met in emergencysituations--and that implies that the system must be able to activelydrop lower priority users also within a very short time frame. And, ofcourse, the ability to quickly and efficiently drop channel assignmentsas soon as channel usage is terminated is also important for efficientusage of the trunked facility even in non-emergency situations.

Prior trunked radio systems have attempted to more or less "just meet"such APCO-16 requirements of timeliness. For example, publishedspecifications of one such prior system indicates an ability to achievechannel update (in a 19 channel system) within 450 milliseconds andchannel drops within 500 milliseconds. To achieve this, it utilizes3,600 bits per second (bps) digital signalling over a dedicated digitalcontrol channel. Unfortunately, although theoretically the APCO-16requirements of timeliness should be met by such a prior system, inreality, the APCO-16 timeliness requirements are often not met--or, aremet only at the expense of suffering with the obviously adverse effectsof somewhat unreliable digital control signalling (which are, at best,annoying even in non-emergency situations). Accordingly, there isconsiderable room For improvement.

The present invention provides substantial improvements--both intimeliness and in reliability of critical control signalling in adigitally trunked radio system of this general type. To begin with, amuch higher digital signalling rate (9600 bps) is utilized. However,rather than using all of the increased signalling rate to provide a9600/3600=2.67 improvement factor in timeliness, a large portion of theincreased signalling rate capacity is utilized to improve signallingreliability. Accordingly, the increased timeliness of 19 channelupdating capability, for example, is improved by a factor ofapproximately 1.58 (e.g., 285 milliseconds versus 450 milliseconds)while the rest of the increased signalling capacity is utilized toincrease the reliability of control signalling. At the same time,virtually all of the increased signalling capacity is utilized toimprove the timeliness of channel drop ability (e.g., 190 millisecondsversus 500 milliseconds).

As previously demonstrated by Bell System Technical Journal articles onthe AMPS system (e.g. "Voice and Data Transmission", by Arredondo et al,The Bell System Technical Journal, Vol. 58, No. 1, Jan. 1979, pp97-122), digital data rates on radio channels should be either very low(e.g., 200 hertz) or as high as thne channel bandwidth permits. Thepresent invention utilizes the maximum high speed data rates (e.g., 9600bps on the typical 25 Kz bandwidth radio channel) for critical controlchannel signalling and o control signalling on the working channels bothimmediately before and immediately after the user communicationinterval. In addition, sub-audible low-speed digital data is alsoutilized on the working channel during user communications so as toassure additional signalling reliability--and to also permitimplementation of additional features.

In the exemplary embodiment, all channels (the control as well asworking channels) are fully duplexed so that there may be simultaneousin-bound and out-bound signalling on all channels. In general, thisinvention achieves reliable and prompt communication within a trunkedradio repeater system having a digital control channel and pluralworking channels, which working channels are assigned for temporary useby individual radio units as specified by digital control signals on thecontrol channel.

Channel assignment is initially requested by a calling radio unitpassing digital request signals to a control site over the activecontrol channel. In accordance with channel availability, a controllerat the central site assigns a specific then-available working channel tothe requested communication and passes digital assignment signalsout-bound over the control channel. Both the calling radio unit and thecalled unit(s) detect the working channel assignment and switch theirtransmitter and receiver operations over to the proper working channel.Thereafter, digital handshake signals are again exchanged between thecontrol site and at least one of the radio units (e.g., the callingunit) over the assigned working channel. In response to a successfulhandshake on the assigned working channel, the central site thentransmits digital release signals over the assigned working channel soas to release the appropriate units for communication thereover.

As one technique for increasing reliability, the initial request signalsmay include three-fold data redundancy (at least for critical data)while the channel assignment signals subsequently transmitted over thecontrol channel may include as much as six fold redundancy of data(e.g., at least of critical data such as that representing the calledparty and the assigned channel). The handshake signals subsequentlyexchanged on the assigned working channel may also include three-folddata redundancy of at least critical data. In this manner, some of theincreased signalling capacity made available by the high-speed data rate(e.g., 9600 bps) is sacrificed in favor of more reliable channelallocation and communication functions--while still comfortablyexceeding all APCO-16 requirements.

To insure responsiveness to higher priority calls, sub-audible digitalchannel assignment update messages are also transmitted over theassigned working channel. These are monitored in each unit then residingon the working channels. Accordingly, if a higher priority call isdirected to some unit already engaged in a communication, that unit isenabled to promptly switch operations to a new assigned working channelso as to immediately receive the higher priority call.

In addition, to accommodate late entry of called parties to an ongoingcommunication, digital channel assignment "late entry" messages continueto be transmitted on the control channel even after a successful channelassignment process has been effected so that late entrants (e.g., thosejust turning on their radio, just passing out of a tunnel or from behinda building or otherwise back into radio communication after temporaryinterruption, completion of a higher or equal priority call, etc.) maynevertheless be switched onto the proper assigned working channel assoon as possible. (The late entry feature, per se, is related tocopending commonly assigned application Ser. No. 725,682 filed 22 Apr.1985.)

To effect prompt and reliable termination of channel assignments, whenthe PTT switch of a calling unit is released, it sends a digital unkeyedmessage on the assigned working channel and, in response to reception ofthis unkeyed message at the control site, a digital drop signal istransmitted over the assigned working channel so as to immediately dropall units therefrom and thus free that working channel for reassignment.(As will be appreciated, a given radio unit will automatically revert tomonitoring the control channel upon dropping from an assigned workingchannel.)

The system of this invention is sometimes termed a "digitally" trunkedsystem because trunking control is effected by digital signals passedover a continuously dedicated "control" data channel. All units areprograrmmed so as to automatically revert to the predetermined primarycontrol channel upon being turned on or being reset. If the expectedcontrol channel data format is not there discovered, then alternatepossible control channels are successively monitored in a predeterminedsequence until an active control channel is discovered. In this manner,the usual control channel apparatus at the central control site may betemporarily removed from service (e.g., for maintenance purposes). Thissame feature also permits continued trunked system operation in theevent that the regular control channel unexpectedly malfunctions or isotherwise taken out of service.

The exemplary embodiment of this invention is designed in such a waythat it meets and, in many areas, surpasses all of the existing APCO-16requirements. It may also support available voice encryption techniques,mobile digital data terminals (digital data may be passed in lieu ofanalog voice data during a trunked radio communication session) and/oravailable automatic vehicular location systems. Preferably, a faulttolerant architecture is used see related copending commonly assignedapplication Ser. No. 87/057,046 GE docket 45-MR-541, filed concurrentlyherewith! so as to maintain trunked system operation even if the centralprocessor happens to fail at the control site. If digital data is to becommunicated between radio units and/or the central site, then it may beprocessed through the system in a manner similar to analog voicesignals. In particular, such digital data communication will be carriedout at a rate accommodated within the existing audio pass band and willbe trunked just like desired voice communications (i.e., no dedicateddigital data communication channels are required). To help increasereliability of digital data communications, data transmissions (andanalog voice transmissions, as well) may be voted in voting systemsemploying satellite receivers connected to the central control site.

In the exemplary embodiment, digital control signalling messages of thefollowing types are utilized:

    ______________________________________    Channel Type              Direction            Rate    ______________________________________    Control Channel              INBOUND              9600 pbs    Group Call    Special Call    Status    Status Request/Page    Emergency Alert/Group Call    Individual Call    Cancel Dynamic Regroup    Dynamic Regroup - Forced Select    Dynamic Regroup - Option Deselect    Dynamic Regroup - Option Select    Login/Dynamic Regroup Acknowledge    Logical ID Request    Programming Request              OUTBOUND             9600 bps    Channel Assignment    Channel Update    Enable/Disable Unit    Dynamic Regroup    Preconfiguration    Alias ID    Unit Keyed/Unkeyed    Emergency Channel Assignment    Cancel Dynamic Regroup    Dynamic Regroup - Forced Select    Dynamic Regroup - Optional Select    Dynamic Regropu - Optional Select    Assign Group ID An Alias ID    Assign Logical ID An Alias ID    Status Acknowledge/Page    Time Mask    Emergency Channel Update    Site ID    System Operationa Made    Site Status    Logical ID Assignment    Programming Channel Assignment    Working Channel              INBOUND              9600 bps    Initial Handshake    Special Call Signalling    Unit ID-PTT and Reverse PTT    Miscellaneous              OUTBOUND             9600 bps    Initial handshake    Channel Drop    Status Messages              INBOUND              Low Speed    Confirm Unit PTT              OUTBOUND             Low Speed    Priority Scan    Falsing Prevention    ______________________________________

Some of the general features of the exemplary embodiment and expectedbenefits are summarized below:

    ______________________________________    FEATURE             BENEFIT    ______________________________________    VERY SHORT AVERAGE  Practically instantaneous    CHANNEL ACCESS TIME access doesn't cut off                        syllables.    Normal Signal Strength                        Provides operation which    Areas: 280 Milliseconds                        is faster than most coded    Weak Signal Areas (12 dB                        squelch systems.    Sinad: 500 Milliseconds    LATE ENTRY          Minimizes missed    Should a mobile turn on                        conversations, keeps    during the period in which                        police up to the minute,    its group is involved in                        minimizes call backs.    a conversation, the mobile    will automatically be    directed to join that    conversation.    AUTOMATIC CHANNEL SWITCHING                        Frequency coordination    Mobiles, portables, control                        of the fleets and    stations, and consoles                        subfleets requires    automatically switch to                        no action on the part of    the appropriate channel.                        any field personnel.    HIGH SPEED CALL PROCESSING                        Dedicated control channel    Processor assigns unit                        provides more rapid    initiating the call, and                        channel assignments on    all called units, to an                        larger systems. System    appropriate working channel.                        size does not impact    Initial channel assignment                        upon channel acquisition    communication between site                        time. Control channel is    controller and radio units                        available for additional    occurs on the control                        functions such as status    channel.            and unit ID.    CALL RETRY          Eliminates the need for    Calling unit will   repetitive PTT operations    automatically repeat its                        by the operator in weak    request up to eight times                        signal situations.    if no response is received.                        Terminating retries also    Retries terminated upon                        shortens the signalling    system response.    time.    UNIT DISABLE        In hostage situations,    Trunked units can be                        these units can be    disabled on an individual                        assigned to a special    basis. These disabled units                        group for communication    continue to monitor the                        with the criminals.    control channel and can be                        Also, with automatic    polled to determine their                        vehicular location,    status.             these units could be                        tracked for apprehension.    SUPPLEMENTARY CONTROL                        Provides user and system    CHANNEL FUNTIONALITY                        operator with system    In addition to providing                        manager features not    channel assignments, the                        available on other types    control channel is used                        of systems.    for: status messages,    polling, system status,    logging, late entry,    dynamic regrouping,    system testing and other    system functions.    GROUP PRIVACY       Each group has the same    Each group hears only his                        privacy as having their    own group, unless   own channel with the    specifically programmed                        additional benefits of    otherwise. Dispatcher                        being regrouped with    can override for individual                        other complementing    units or groups of mobiles                        functions for emergency    at any time.        operations.    CALL QUEUING        Maintains orderly entry    When all channels are                        procedure for busy    busy, call requests will                        system. Call requests    be queued until a channel                        are accepted in the order    becomes available. Unit                        they are received except    requesting a channel will                        higher priority users go    be notified to prevent                        to the head of the line.    call backs. Members of    groups already in the queue    will not be reentered in    the queue.    DATA COMMUNICATIONS This feature greatly    System has the optional                        enhances the value of the    capability of using 9600                        system because it avoids    bps data on the working                        the expense of additional    channels. Data      RF channels.    communications will take    place on any equipped    working channel and they    are trunked, just as    voice communication.    VOICE ENCRYPTION    Voice encryption offers    System has the optional                        the same encrypted range    capability of using as for clear voice    available 9600 baud voice                        transmissions. Voice can    encryption. (See, e.g.                        be passed from each site    commonly assigned U.S.                        through conventional    Pat. No. 4,622,680  voice grade phone lines    and aplication Ser. or microwave links. Only    Nos. 661,597 filed 17                        minor modifications are    October 1985, 661,733                        needed to the base    filed 17 October 1984                        station interface    and 661,740 filed 17                        equipment to accommodate    October 1984.)      such sophisticated voice                        security systems.                        Any mobile can be                        upgraded to this system                        capability by adding an                        external module. No                        internal changes to the                        radio are required.    UNIT IDENTIFICATION Each unit on each    All units are automatically                        transmission is    identified when they                        identified by the same    transmit. This is true                        ID regardless of the    regardless of whether the                        agency, fleet or    transmission takes place                        subfleet in which he is    on the control channel                        currently operating.    or on a working channel.                        4095 is more than twice    The system is able to                        the logical number    accommodate 4095 discreet                        of users in a fully    addresses independent of                        loaded twenty channel    fleet and subfleet. system so there is more                        than sufficient capacity.    TRAFFIC LOGGING     Statistics on system    All system information is                        usage, such as peak    logged. Each unit trans-                        loading, individual and    mission causes the units                        group usage versus time,    ID, agency, fleet, sub-                        as well as many other    fleet, channel, time, site                        system parameters, are    and list of sites involved                        available for tabulation    to be logged for manage-                        and analysis.    ment reports.    TELEPHONE INTERCONNECT                        All mobiles and portables    Authorized mobiles have                        in the system can be    the ability to place and                        interconnected to the    receive calls and patch                        telephone system. Those    them to individuals or                        mobiles not specifically    groups of mobiles which                        equipped for this can be    may or may not be equipped                        patched by their dis-    for telephone interconnect.                        patcher to maintain                        adequate control of                        system loading factors.    GROUP PRIORITY ASSIGNMENT                        System manager can set    Eight priority levels are                        individual group    provided in the system.                        priorities according to    Each group (as well as                        the criticality of their    each individual) is service.    assigned a priority.                        The flow of traffic is                        more easily maintained                        by providing recent users                        priority over nonrecent                        users of the same                        priority level.    ______________________________________

In the exemplary system, 11 bits are available to determine the addressof a unit within an agency, fleet or subfleet. Twelve bits are availableto determine the individual identity of a particular unit (i.e., its"logical ID"). The use of 11 bits for determining group addresses withinan agency, fleet or subfleet provides great flexibility such that eachagency may have many different fleet and subfleet structures.Furthermore, unit identification is not limited by a particular fleetand subfleet structure. The 4,096 unit identification codes can bedivided among the subfleets in a manner that best suits a particularsystem.

Some features of this exemplary system which are believed to beparticularly unique and advantageous are summarized briefly below (orderof appearance not reflecting any order of importance--nor is this listto be considered in any way exhaustive or limiting):

a) Widening the Retry Window

If a requested working channel assignment is not achieved, the requestis automatically retried--and the time window in which such a retry isattempted is increased in duration as a function of the number of priorunsuccessful retries. This significantly decreases the average channelaccess time where noise is the real problem rather than requestcollisions--while still providing a recovery mechanism for requestcollision problems as well.

b) Better Use of Subaudible Signalling

Rather than using subaudible signalling only to confirm channelassignments, a simple counter field is employed to greatly simplify suchvalidity checking functions and to thus free the majority of thesubaudible signalling capacity for other uses--e.g., a priority scan. Inthe exemplary embodiment a two bit subaudible "count" field for a givenchannel is incremented upon each new working assignment of that channel.Thus, if a radio unit observes a change in this field, it is programmedto immediately drop back to the control channel.

c) Minimizing Priority Communique Fragmentations by Dynamically AlteringScan Functions

After initiating a priority call, a radio temporarily (e.g., for twoseconds) disables the usual multiple group scan on the controlchannel--in favor of looking for the highly probable returned higherpriority call. This reduces the possibility of getting divertedmomentarily into an ongoing lower priority communique--and also perhapsmissing a fragment of the next higher priority communique. A similartemporary (e.g., two seconds) scan preference (except for prioritycalls) for a just-previously involved call group also helps preventfragmentation of non-priority communiques.

d) Use of Transmission-Trunked Bit in Channel Assignment

The trunking system has two trunking modes:

a) Transmission Trunked Mode in which the working channel isde-allocated as soon as the calling unit unkeys, and

b) Message Trunked Mode in which the working channel is de-allocated "n"seconds following a unit's unkeying, unless another unit keys onto thechannel within such "n" seconds. "n" is called the "hang time".

By dynamically insuring that both called and calling units "know" that atransmission-trunking mode is in effect, the calling unit mayimmediately revert to the control channel upon PTT release--thusimmediately freeing the working channel for drop channel signalling fromthe control site. The called units can also be positively prevented fromever transmitting on the working channel--thus avoiding multiple keyingof radio units on the working channel.

e) Automatic Addressing of Immediately Returned Calls

Both the called and calling units/groups are identified in the initialchannel assignment signalling. The called unit captures the calling unitID and is enabled to automatically address a return call to the justcalling radio if the PTT switch is depressed within a predeterminedperiod (e.g., 5 seconds) after the just completed communique even if thesystem is in the Transmission Trunked Mode. Not only does this simplifythe necessary call back procedures and minimize access times, byallowing greater application of the Transmission Trunked Mode it alsoincreases the probability of successful message exchanges--especially inpoor signalling areas.

f) 9600 bps Permits "Loose" Synchronization

Use of higher rate 9600 bps signalling permits simplified bitsynchronization to be rapidly achieved by simple "dotting" sequences(i.e., a string of alternating ones and zeros 101010 . . . ). Thus,there is no need to keep information transfers precisely synchronizedacross all channels. This not only reduces hardware requirementssystem-wide, it also facilitates a more fault tolerant architecture atthe control site.

g) Improved Channel Drop Signalling

The drop channel signalling is simply an extended dotting sequence.Therefore, each radio may easily simultaneously look for drop channelsignalling and channel assignment confirmations. This means that thecontrol site may more immediately consider a given working channelavailable for reassignment--and, if "loaded up," immediately interruptthe drop-channel signalling to issue fresh channel assignmentconfirmation signals on the working channel (which each individual radiowill ignore unless properly addressed to it). As a result, a "loaded"system (i.e., one where existing channel requests are already queued)may drop a working channel within about only 100 msec--and immediatelyreassign it to a queued request. Radios that happen to enter late intothe call being dropped can detect that fact and properly drop from thechannel because of the ability to simultaneously look for drop channelsignalling and channel assignment confirmation signalling.

h) Feature Programming

To avoid cumbersome feature programming (and reprogramming to addfeatures) by factory or distribution personnel, novel procedures areemployed which safely permit the end user to perform all such"programming." All units are programmed at the factory to perform allavailable functions. A function enable bit map and a unique physical IDare together encrypted at the factory and provided to the user as"Program Codes." When the user programs each device, its encrypted"Program Codes" are input to a Radio Programmer which, in turn, properlysets the feature enable bit map in a connected radio unit--and thedecoded physical ID--and a "Just Programmed" bit). The "just programmed"radio device logs into the central controller with a request for alogical ID--based on its apparent physical ID. If illegal copying offunction enabling Program Codes occurs, then the same logical ID will beassigned--and the usefulness of the radio within the trunked repeatersystem will be diminished.

i) Double Channel Assignment Handshake--One Being on the AssignedWorking Channel

A first 9600 bps channel assignment signalling exchange occurs on thecontrol channel. However, a confirmation (i.e., a second handshake) thenoccurs on the assigned working channel. Thus, it is assured that thedesired channel has been successfully assigned and locked onto beforethe central controller unmutes the called units on the assigned channel.The signalling is such that if the channel conditions are unsuitable forvoice, the handshake will fail, thus terminating the call automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

These as well as other objects and advantages of this invention will bemore completely understood and appreciated by carefully studying thefollowing detailed description of the presently preferred exemplaryembodiment taken in conjunction with the accompanying drawings, ofwhich:

FIG. 1 is a general explanatory diagram of trunked radio repeater systemin accordance with this invention;

FIG. 2 is a simplified block diagram of a central control site (as wellas a satellite receive site) in the trunked repeater system of FIG. 1;

FIG. 3 is a simplified block diagram of the general site architecture ofthe main controller for the central control site;

FIG. 4 is a simplified block diagram of the channel architecture usedwithin each channel of the central site architecture shown in FIG. 3;

FIG. 5 is a simplified general block diagram of a technicalmobile/portable radio unit to be utilized for communication within thetrunked repeater system of FIG. 1;

FIG. 6 is a simplified flowchart of typical call processing sequence inthe exemplary embodiment from the perspective of a calling unit;

FIG. 7 is a simplified flowchart of a call processing sequence within acalled unit;

FIG. 8 illustrates the trunked channel dropping procedure and typicalrequired drop times;

FIG. 9 generally illustrates call initiation signalling within theexemplary system as well as typical timing requirements;

FIGS. 10A and 10B generally illustrate the control signalling protocolutilized to initiate and terminate trunked radio communications oneither an individual or group basis in the exemplary system; and

FIGS. 11A and 11B are flowcharts of suitable computer programs whichmight be utilized at the site controller, the calling unit and thecalled unit so as to achieve the signalling protocol of FIG. 10.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

An exemplary trunked radio repeater system in accordance with thisinvention is generally depicted at FIG. 1. As illustrated, individualunits of various groups communicate with each other (both within andpossibly outside of their own group) via shared radio repeater channelslocated at a trunked repeater control site 100. The dispatch console 102may be housed directly at the repeater station site 104 or may beremotely located via other communication facilities 106 as will beappreciated by those in the art. There also may be multiple dispatchconsoles 102 (e.g., one for each separate fleet) and a master orsupervisory dispatch console for the entire system as will also beappreciated by those in the art.

The central site is depicted in somewhat more detail at FIG. 2 inconjunction with one or more satellite receiver sites 100-1. As will beappreciated, the satellite receiver sites are displaced spatially fromthe central site 100 such that radio reception may temporarily be betterat one or the other of the chosen antenna sites. Thus, received signalsfrom the satellite sites as well as the central sites are combined in"voter" circuitry so as to choose the best available signal for controlor communication processes.

At the central site, a transmitting antenna 200 and receiving antenna202 (which may sometimes be a common antenna structure) may be utilizedwith conventional signal combining/decombining circuits 204, 206 as willbe apparent to those in the art. The transmitting and receiving RFantenna circuitry 200-206 thus individually services a plurality ofduplex RF channel transmit/receive circuits included in a plurality ofRF repeater "stations" 300, 302, 304, 306, etc. Typically, there may be20 such stations. Each station transmitter and receiver circuitry istypically controlled by a dedicated control shelf CS (e.g., amicroprocessor-based control circuit) as is also generally depicted inFIG. 2. Such control shelf logic circuits associated with each stationare, in turn, controlled by trunking cards TC (e.g., furthermicroprocessor-based logic control circuits) 400, 402, 404 and 406. Allof the trunking cards 400-406 communicate with one another and/or with aprimary site controller 410 via control data bus 412. The primary sitecontroller (and optional backup controllers if desired) may be acommercially available general purpose processor (e.g., a PDP 11/73processor with 18 MHz-J11 chip set). Although the major "intelligence"and control capability for the entire system resides within controller410, alternate backup or "fail soft" control functions may also beincorporated within the trunking cards 400-406 so as to providecontinued trunked repeater service even in the event that controller 410malfunctions or is otherwise taken out of service. (More detail on suchfail soft features may be found in commonly assigned concurrently filedapplication Ser. No. 07/056,046 (GE docket 45-MR-541) entitled "FailSoft Architecture For Public Trunking System".)

An optional telephone interconnect 414 may also be provided to thepublic switched telephone network. Typically, a system manager terminal,printer, etc., 416 is also provided for overall system management andcontrol (together with one or more dispatcher consoles 102). A specialtest and alarming facility 418 may also be provided if desired.

The signal "voter" circuits 502, 504, 506 and 508 are connected so as toreceive a plurality of input digital or analog signals and toselectively output therefrom the strongest and/or otherwise mostreliable one of the input signals. Thus, received signals from thecentral site 100 are input to respective ones of the channel votercircuits 502-508 while additional similar input signals are generatedfrom receivers in the satellite receiver site 100-1 and also input tothe appropriate respective voter circuits. The results of the votingprocess are then passed back to the trunking card circuits 400-406 wherethey are further processed as the valid "received" signals.

A slightly more detailed view of the site architecture for control datacommunication is shown in FIG. 3. Here, the PDP 11/73 controller 410 isseen to communicate over a 19.2 Kilobit link 412 with up to 25 trunkingcontrol cards TC controlling respective duplex repeater circuits inthose individual channels. Another high-speed 19.2 Kilobit link 420 isused to communicate with the hardware that supports the down linkto/from the dispatch console 102. Other data communication with thecentral processor 410 is via 9600 baud links as shown in FIG. 3. Thecentral processor 410 may include, for example, a 128 Kilobyte codePROM, 1 Megabyte of RAM and 32 DHV-11/J compatible RS-232C ports. It maytypically be programmed using Micropower Pascal to provide amulti-tasking, event-driven operating system to manage all of thevarious data communication ports on an acceptable real time basis.

At each controlled repeater channel, the 19.2 Kilobit data bus 412 (aswell as that from an optional back-up controller if desired) ismonitored by an 8031 processor in the TC module. The TC trunking controlmodule exercises control over the control shelf CS of its associatedrepeater with audio, signalling, and control busses as depicted in FIG.4, and may typically also receive hard-wired inputs providing clocksynchronization and a "fail soft" indication (e.g., indicating thatnormal control by the central controller 410 is not available and thatan alternate distributed control algorithm should then be implementedwithin each of the trunking control modules TC).

The general architecture of a suitable mobile/portable radio unit foruse within the exemplary system is also microprocessor based as depictedin FIG. 5. Here, microprocessor 550 is provided with suitable memory 552and input/output circuits 554 so as to interface with the radio unitdisplay, keypad, push-to-talk (PTT) switch as well as audio circuits 556which provide basic analog audio outputs to the speaker and acceptanalog audio inputs from the microphone. Auxiliary control over a modem558 as a digital interface (e.g., to voice encryption, vehicle locationor other types of digital communication subsystems) may also be providedif desired. And, of course, the I/O circuits 554 also permit suitableprogrammed control over RF receiver 560 and transmitter 562 which, viaconventional signal combiners 564 permit two-way fully duplexedcommunication over a common antenna 566 as will be appreciated by thosein the art.

A detailed and indepth description of all units and sub-units of such asophisticated system would necessarily be extremely voluminous andcomplex. However, since those in this art are already generally familiarwith digitally controlled trunked repeater systems with suitable RFtransmitter and receiver circuits, programmed general purpose computercontrollers, etc., no such exorbitantly detailed description is believednecessary. Instead, it would only serve to obscure the subject matterwhich constitutes the invention. Accordingly, the remainder of thisdescription will concentrate on the signalling protocol utilized toinitiate and terminate calls within the system since this is believed toconstitute a significant improvement (both in reliability andspeed)--while still facilitating the retention of many highly desirablesystem features and meeting or exceeding all APCO-16 requirements.

Call placement begins by the calling unit transmitting a special digitalchannel request signal on the dedicated control channel to the centralsite. In return, the central site transmits, outbound on the controlchannel, a special digital channel assignment signal. The calling unitthen responds by switching immediately to the assigned working channelwhere the central site now sends an assignment confirmation message(also in high-speed digital form). If the calling unit properly receivesthe confirming signals on the working channel, then it responds with anacknowledgment back to the central site on the working channel tocomplete the second handshake (i.e., the first one was on the controlchannel and now one has taken place on the working channel) before thecentral site releases the called units to begin the requestedcommunication session on the working channel. Alternatively, if duringthis process the calling unit receives a channel update message on thecontrol channel addressed to it, then the channel request call istemporarily suspended (unless the channel request under way is anemergency or higher priority request) and the calling unit then revertsto the called state so as to receive the incoming call. If the callingunit receives no response (or an improperly completed response handshakesequence), it automatically waits a random period before retrying tosuccessfully place the call request (up to a maximum of 8 tries).

The called unit initially resides in a standby configuration where itcontinually monitors the digital messages appearing on the controlchannel outbound from the central site. If it detects a channelassignment message addressed to it as the called party (or perhaps asone party of a called group), then the called unit immediately switchesits operations onto the assigned working channel. There, it also detectsthe confirmation signals outbound on the working channel from thecentral site and, if successfully confirmed, awaits a release orunsquelching signal on the working channel (e.g., transmitted from thecentral site in response to successful completion of a handshake withthe calling unit on the working channel). The called unit may alsoreceive a channel update message indicating that the group i is alreadyoperating on a working channel.

The programming for a calling unit is generally depicted in thesimplified flowchart of FIG. 6. Here, upon entry into the calling mode,a call request is sent on the control channel CC at step 600. A test ismade at 602 for call queuing. If queued, transfer is made to wait loop603 (including a test for a detected assignment at step 604 followed bya check for expiration of a 30 second timer at 606 (whereupon controleffectively is passed back to a manual requirement to restart thecalling process via exit 607).

If the call request is not queued, then a test is made at 608 to see ifthis particular unit has already previously been requested as a calledparty. If so, then transfer is made at 610 to the called mode ofoperation. If not, then a check for a returning channel assignment ismade at 612. If not received at the expected time, then a random wait isinterposed at 614 before a test is made at 616 to see if eight trieshave yet been made to complete this particular call. If so, then thesubroutine is exited at 618. If not, then the subroutine is re-enteredat 600.

If a channel assignment is successfully detected at either 612 or 604,then the unit operation is immediately switched to the assigned workingchannel at step 620 and a test for the second successful handshake(confirmed signalling) is made at 622. If unsuccessfully confirmed onthe working channel, then exit will be made and the call is terminated.However, if the second handshake (e.g., the handshake on a workingchannel) is successfully confirmed and completed, then the calling unittransmits an elongated sequence of dotting at 624 (e.g., representingthe successful second handshake) followed by a transmission of voice at626 (or data if a digital communication session has been requested) overthe assigned working channel before exit from the subroutine is taken at628 (e.g., to a standard monitoring routine which looks for release ofthe PTT switch and transmits an unkeyed signal at 627).

The protocol followed by a called unit is generally depicted at FIG. 7(e.g, representing a suitable computer program for controlling the unitin this mode of operation).

Upon entry, the control channel is simply monitored at 700 for any"good" message (e.g., one addressed to this particular unit). If such amessage is detected, a check is made at 702 for an "update" type ofmessage. If the message is of this type, then a check is made at 704 tosee if it is repeated within about 1.0 seconds. If not, then reentryinto the called mode is made. However, if the update of a higherpriority incoming call is repeated within such period, then an immediateswitch to the there-assigned working channel is made at 706. Ifsignalling is not confirmed at 707, then an immediate switch tounsquelching (716) is made and that channel is thereafter monitored. If,on the other hand, signalling confirmation at 707 is achieved, it is anindication that normal channel assignment is, in reality, taking placeand control is passed to block 714 to look for an unmute message.

If no channel update message is detected at 702, then the message ischecked to see if it was a channel assignment at 708. If not, thenreturn is made to the beginning of the subroutine. However, if a properchannel assignment has been received, then a switch to the assignedworking channel is made at 710 and a check for proper confirmationsignalling on the working channel is then made at 712. If a properurmuting message is thereafter also received on that assigned workingchannel at 714, then the called unit unsquelches at 716. If no unmutingmessage is received at 714, then a check for a drop message is made at718. If there is no drop message but high-speed signalling is stillpresent on the working channel (as detected by 720), then a furthercheck is made for the unmute message at 714. However, if there is nodrop message and the high-speed signalling has ceased at 720, then thecalled unit is nevertheless unsquelched at 716.

At the conclusion of a desired audio call, the calling radio transmittertransmits a special release PTT signal as depicted graphically in FIG.8. After a suitable transmission and detection delay period, theassigned working channel responds by transmitting a drop channel signalon the working channel. As shown in FIG. 8, this results in a typicalworking channel availability in only 167 milliseconds after the releasePTT signal is initiated.

Typical timing of calling protocol signals is depicted graphically inFIG. 9 where in combination with Table T1 it can be seen that typicalcalling protocol can be completed and communication begun over thedesired working channel within about 290 milliseconds.

                  TABLE 1    ______________________________________                       MIN   TYP     MAX*                       (msec)                             (msec)  (msec)    ______________________________________    t.sub.s =         TIME TO SYNC & START TX                             30      45    60    t.sub.tx =         TX TIME             30      30    30    t.sub.w =         WAIT TIME           30      60    60    t.sub.ca =         CHANNEL ASSIGNMENT TIME                             30      30    30    t.sub.wc =         TIME TO WORKING CHANNEL                             15      20    25    t.sub.c =         TIME TO CONFIRMATION                             25      25    60    t.sub.cd =         TIME BETWEEN CONFIR-                             20      20    25         MATION & DOTTING    t.sub.d =         TIME FOR SENDING DOTTING                             60      60    60    t.sub.u =         TIME TO UNMUTE       0       0    30                             240     290   380    ______________________________________     *MAXIMUM TIME ASSUMING SUCCESS

Some bit-level maps of some relevant message formats (and other relatedsignalling formats and protocol) are graphically illustrated at FIG. 10Aand 10B. The control channel transmits an outbound continuoustransmission repeating the format 800 depicted in FIG. 10A. As will beseen, each 40 bit message is transmitted three times (including oneinverted transmission where all 0's are changed to 1's and vice versa)and there are two such messages transmitted per recurring message time"slot." As will be appreciated, the optional dotting prefix (if used)insures continued bit synchronization by receiving units and the uniqueBarker code permits frame synchronization so as to define bit boundariesbetween the 40 bit-level messages which follow. Since the controlchannel transmits these message slots continuously, no dotting prefix isneeded and but one transmission of the word framing Barker code willsuffice for each recurrent transmission cycle. Of course, if desired, arelatively short dotting prefix may be used to even further insurecontinued bit synchronization.

Inbound messages on the control channel CC are of the format 802 shownin FIG. 10A and may comprise, for example, group/individual channelassignment requests transmitted from a calling unit. Here, the dottingprefix is considerably longer and the word framing Barker code isrepeated three times so as to insure that the receiving circuits at thecentral site are properly synchronized before the 40 bit messages (againwith three-fold redundancy) are transmitted. Preferably, suitabletransmission timing circuits are utilized so as to make such incomingcontrol channel messages synchronously time "slotted"--meaning that themessages on the inbound portion of the control channel occur during thesame time slot as outgoing messages from the central site on the controlchannel (as generally indicated by dotted lines in FIGS. 10A and 10B).

A group call request message format is shown in expanded scale in FIG.10A. It includes a two-bit message type (MT) code (the message-typefield may be extended in a tree-logic fashion to include additional bitsas will be appreciated). This MT-A field thus distinguishes a group callfrom an individual call, for example. A type of communications fieldcomprising two-bits indicates the type of communication session beingrequested. (If desired, a priority field of one-bit also may be used toindicate if a highest priority emergency call is being requested.) Thecalled identification code of 11 bits (representing either a group or anindividual unit) is followed by a 12-bit field representing the identityof the calling unit ("logical ID"). The 40-bit message concludes with 12bits of standard BCH code for error detection and correction purposes aswill be appreciated.

The returning channel assignment message actually comprises a twomessage pair also having a format as shown in expanded scale at FIG.10A. The first two bits identify the message type (MT) and the next twobits identify the type of communication session which is to take place.The identity of the calling unit is next represented by a six-digitfield (e.g., with the 6 most significant bits being transmitted in onemessage of the two-message pair while the 6 lowest significant bits aretransmitted in the other of such messages). The next one-bit fieldidentifies whether a group call or an individual call is involved andthe assigned working channel is identified by the following 5 bits. Thegroup or individual identity of the called unit(s) is contained withinthe next 12 bits followed by 12 bits of BCH error detection/correctioncode.

Once operation reverts to the assigned working channel, the central sitetransmits a confirmation message of format 804 outbound on the workingchannel. As will be observed, it is of the same general form as thecontinuous transmissions on the control channel CC except that themessage length has been reduced to 32 bits on the working channel. Onceagain, the message is sent with three-fold redundancy (one beinginverted). Preferably, the confirmation message is timed in the workingchannel so as to be within the same time slot as messages beingtransmitted on the control channel. The format of the 32 bitconfirmation message is also depicted in expanded form at 806 in FIG.10A. Here, 4 bits are devoted to the message type code while 2additional bits provide a subaudible frame count useful in framing andotherwise decoding the lower speed subaudible digital data (which willsubsequently appear on the working channel to be monitored by unitsresiding thereat). One bit is also devoted to identifying thecommunication session as one which is transmission trunked or one whichis message trunked. Another bit of the confirmation message 806identifies the call as being either of a group or an individual unitwhile the identity of the called group or individual unit is containedwithin the following 12 bits. The confirmation message 806 concludeswith 12 bits of BCH error detection/correction code.

Once the second handshake (i.e., on the working channel) has beensuccessfully concluded, the calling unit transmits 384 bits of dottingfollowed by audio (in the case of a requested audio communicationsession) as is also depicted a. FIG. 10B.

The elongated dotted sequence transmitted by the calling unit on theworking channel constitutes an acknowledgment of the successfulhandshake sequence and, in response, the central site transmits anoutbound digital message on the working channel to positively unmute thecalled unit(s). The format 808 of such an unmute message is depicted inFIG. 10B. Once again, the message type code uses the first four bitswhile a subaudible frame count constitutes the next two bits. The nextbit denotes trunked status or non-trunked status (e.g., regularhang-time) while the next bit is effectively unused (e.g., preset tozero in all unmute messages)--but which may be used for other optionalpurposes. The identity of the unit(s) to be unmuted is set forth in thenext 12 bits followed by 12 bits of standard BCH errordetection/correction code.

At the conclusion of a communication session on the working channel, thecalling unit again transmits 384 bits of dotting followed by 4 datablocks of 128 bits each. Each such data block includes 16 dotting bitsand a 16 bit Barker code (some of which bits may be "filler" as will beappreciated) as prefix followed by 8-bit bytes, each of which istransmitted with three-fold redundancy (one inverted)--thus constitutinga 32-bit message characteristic of digital messages being transmitted onthe working channel. The format of the 32-bit unkey message 810 is alsoshown in FIG. 10B. Here, a 4-bit message type code is followed by 2unused bits and 2 bits for a block count. The identity of the callingunit is set forth in the next 12 bits followed by 12 bits of standardBCH error detection/correction code.

Finally, in response to receipt of the unkey message at the central siteon the working channel, an outbound digital message on the workingchannel of a super-extended dotting sequence (e.g., 896 to 2816 bits) istransmitted from the central site as depicted at 812 in FIG. 10B--and inresponse, all units then on that channel drop from that particularworking channel and revert to the active control channel.

The sentence of programmed events occurring at the site controller, thecalling unit and the called unit(s) during a typical callorigination/termination sequence is depicted in the parallel flowchartsof FIGS. 11A and 11B.

Each programmed unit has a quiescent control channel (CC) monitorroutine where, in a quiescent state, all units and the site controllerreside. When the calling unit enters the calling subroutine from the CCmonitor at 1100, a test is made at 1102 to see if this calling attemptis a retry. If not, the retry counter is set to a maximum content of 8at 1106 and then decremented by one at 1108 (which step is directlytaken from test 1102 if a retry is in progress). If the retry counterhas been decremented to zero as tested at 1110, then a failedacquisition audible beep is generated at 1112 and exit is taken back tothe CC monitor. On the other hand, if the maximum number of retries havenot yet been made, then a channel assignment request is transmitted onthe control channel and slot synchronization at 1114 (e.g., at time t₁).

Upon detecting an inbound message, the site controller will receive andstore the channel assignment request and assign a free working channelat step 1200. In the exemplary system, a response to an inbound requestmay be supplied within a predetermined delay. The outbound channelassignment messages (i.e., a message pair) are transmitted on thecontrol channel as soon as possible at step 1204 (time t₂). The twomessage channel assignment pair is then received and stored from thecontrol channel in the calling unit at step 1118 (the unit will look forthe messages up to the maximum number of slots). If either message ofthe two message pair is successfully received, this will suffice. Aspreviously explained, if a channel update is received in the interim,then an exit may be taken to a called state (assuming that the callrequest under way is not an emergency). If a valid channel assignmentmessage has not been received as tested at 1120 and the maximum numberof slots have been observed, then a suitable delay is loaded at 1122 anexit is taken back to the CC monitor (from which a return entry to thecalling subroutine will soon be taken).

The process of loading in a suitable delay before retrying may bethought of as a progressive "widening" of the retry window--in aconsciously controlled manner. There are three reasons why an inbounddata message from a radio would not get a response: (1) the inboundmessage was not successfully detected; (2) the outbound message was notsuccessfully detected; or (3) a collision occurred (two or more mobilessent in a request on the same inbound control channel slot).

Given that a collision has occurred, unless mobiles randomly retransmittheir requests, collisions will continue to occur. Consequently, when aradio fails to receive a response to an inbound message, it waits a"random" period of time to retransmit its request. However, if case (1)or (2) has occurred, there is really no reason to randomize the retry.Unfortunately, the radio cannot determine the cause of a failedresponse.

But, the longer a mobile waits to retransmit, the longer the averageaccess time becomes in poor signalling areas since that is where themajority of retries take place. Since often it is noise and notcollisions that cause missed responses, randomizing retries is oftenwasteful.

To correct this problem, the present invention takes some correctiveaction. First, non-channel-acquisition messages are caused to have amuch slower retry rate than channel request messages. Access time forthe former is not critical (whereas it is for the latter). So, if acollision occurs between a radio sending in a non-channel requestmessage and a radio sending in a channel request message, the former'sretry rate will be slow enough to guarantee no chance of a collisionwith the latter on the next retry.

Second, the random retry rate varies with the retry number. Te retryalgorithm (for channel acquisition messages only) widens the width ofthe retry window with each succeeding retry. This decreases the averageaccess time in the presence of noise but still provides a recoverymechanism should the cause for a missed response be a collision.

The preferred embodiment uses the following simple rule:

    ______________________________________    1st retry         2 slot random variability    2nd retry         4 slot random variability    succeeding retries                      8 slot random variability    ______________________________________

It also is possible to vary the retry window width as a function of thereceived bit error rate in order to gain still greater efficiency.

If a valid working channel assignment has been received as tested at1120, then the calling unit switches immediately to the assigned workingchannel at 1124 and waits to receive a proper confirmation message onthe working channel at 1126--which confirmation message is beingtransmitted by the site controller at stem 1206 at time t₃. If theconfirmation message is overridden by a drop message as tested at 1128or by timeout of a preset timer at 1130, then the calling routine isaborted and return is taken to the CC monitor. On the other hand, if aproper confirmation message is received at 1126, then the calling unitbegins to transmit 384 bits of dotting on the working channel at 1132followed by voice transmissions (or other desired communication session)at 1134.

Back at the site controller, a check is made at 1208 for theacknowledgment dotting of extended duration on the working channel. Ifit is not received, then exit is taken. However, if it is properlyreceived, then two unit-keyed/unmute messages are transmitted outboundon the working channel at step 1210.

While all of the above has been taking place, the called unit has (ifeverything is working properly) received and stored the two-messagechannel assignment pair from the control channel at time t₂ (atsubstantially the same time as the calling unit) as depicted at 1300.(Once again, seeing either message of the two message pair issufficient.) In response, the called unit is also switched to theassigned working channel at 1302 and has thereafter monitored theassigned working channel for the proper confirmation thereon at step1304 (and at time t₃. Only if the proper confirmation message has beenreceived does the called unit then look for and receive the unmutemessage transmitted from the site controller on the working channel attime t₅ and, in response, unmutes the receiver of the called unit on theworking channel at 1306.

During the ensuing communication session on the assigned working channelbetween the calling and called unit(s), the site controller (via theTC's) continues to send subaudible new channel assignment (and drop)data to all units on all working channels at 1212 (thus enabling higherpriority calls to be promptly received and accepted by all units). Thesite controller (via the proper TC) also continues to transmit channelupdate messages periodically on the control channel at 1214 (e.g., so asto permit late entrants to immediately go to the proper workingchannel). The site controller informs all TC's of the channelassignments and drops and, in response, each TC generates suitablesubaudible signalling for its channel.

In existing systems subaudible signalling typically is used as avalidity check by mobiles. When a mobile is on a working channel itmonitors the subaudible signalling to make sure it belongs on thatchannel. There are at least two reasons why a radio could get onto achannel where it does not belong:

1) being correctly within a communique on a working channel,it fails tosee the channel drop; or

2) monitoring the control channel, it incorrectly decodes a message andgoes to an incorrect channel.

Problem (1) is solved by giving a two bit subaudible count to all radioson the channel. Every time a call is placed on a channel, the channel TCincrements its count. Consequently, if a radio sees the count change, it"knows" it missed a channel drop sequence.

As for problem (2), there is a sufficiently high probability ofincorrectly decoding outbound control messages on existing systems thata quick way to redirect radios from channels where they do not belong istypically provided. To do this, subaudible signalling typically is usedexclusively for this purpose. However, with this invention, advantage istaken of the high information rate on the control channel, and a mobileis required to see an update message twice before going to a workingchannel. There is a negligible increase in the late entry time, but theprobability of going to an incorrect channel is virtually eliminated. Asa result, subaudible data can also be used for another purpose . . .e.g., a priority scan.

At the conclusion of the desired communications session, the unkeying ofthe PTT switch in the calling unit is detected at 1136 resulting in thesending of an unkeyed message on the working channel at 1138 (time t₆).If in a transmission-trunked mode, the calling unit may immediatelyrevert to the control channel--thus immediately freeing the workingchannel In response, at 1216, the site controller receives the unkeyedmessage on the working channel and, at 1218, sends a super elongateddotting string (896 to 2816 bits on the working channel at time t₇). Thecalled unit has, of course, also received the unkeyed message on theworking channel at time t₆ and, in response, has already muted thereceiver at 1308. The called unit receives the super-elongated dottingstring outbound from the site controller on the working channel at timet₇ and, in response, reverts to the control channel at 1310.

A special priority scan sequence is used (in the preferred embodiment)to minimize communique fragmentation.

When a radio unit scans for multiple groups and a call is made to itspriority group, the radio automatically disables the multiple group scan(in favor of a priority group only scan) for a two second interval uponreturning to the control channel. Since the priority group was justcommunicating, the probability is high that another communique will takeplace within this interval. If the radio immediately scanned intoanother (non-priority) group call (which by definition is of a lowerpriority), and another communicate then occurs on the priority group,the radio would hear a communique fragment from a non-prioritygroup--and would have its entry into the next priority group communiquedelayed (priority scan typically may take between 1.0 and 1.5 seconds toget the radio into the priority group).

Another unique feature used to minimize communique fragmentation is apreference priority the radio automatically assigns to a just-previouslymonitored non-priority group. In essence, if a non-priority communiqueis monitored, for two seconds following the communicate the radio willignore all other scanned calls (except to the priority group of course). . . Similarly to priority group communiques. In addition, the radioalways remembers the last non-priority group monitored. Upon returningfrom a priority group communique, the radio will prefer the lastnon-priority group monitored over any other groups being scanned.

In the example below, a `--` means the group is involved in a communiquechannel, and it is assumed that Group A is the priority group and thatits communiques are separated by less than 2 seconds:

    ______________________________________    Group A    ----    Group B    Group C    ------------------------------------    Radio  BBBAAA   AAAA    AA   AAAAA  BBBB  CCCC    monitors:    ______________________________________

There is a bit (i.e., the message/transmission trunked bit) in theworking channel confirmation signalling that informs the radios as towhether the communique is transmission-trunked or message-trunked. Thisunique feature offers greater frequency efficiency.

The calling radio will be on the working channel and is guaranteed tosee the message/transmission trunked bit. If the bit is set to"Transmission Mode," the calling mobile knows the channel will beremoved as soon as it stops transmitting. Consequently, when its PTT isreleased, the calling radio automatically and immediately goes back tothe control channel. This gains channel usage efficiency because theworking channel TC can begin channel drop signalling as soon as itdetects the calling mobile's unkey message. That is, one does not haveto extend the signalling to make sure the transmitting mobile finishestransmitting, gets its receiver on channel and then has plenty of timeto be guaranteed that the channel drop signalling is detected in thecalling mobile.

Radios that are called also look at this message/transmission trunkedbit, but for an entirely different reason. If the communique ismessage-trunked, radios that were called must be able to key on theassigned working channel in case they must offer a response before thechannel drop. However, if the communique is transmission-trunked, noneof the called radios should ever transmit on the assigned workingchannel. Therefore, if the bit is set to "Transmission" mode, calledradios will not be permitted to key on the working channel. This is avery useful feature since it prevents radios from keying on top of eachother.

The message/transmission trunked bit thus offers three systemadvantages: It makes transmission trunking more frequency (i.e.,channel) efficient by decreasing the channel drop time (by a factor ofthree from typical prior systems), it reduces. the dead time betweentransmission where users can not key (e.g., on typical existing systems,if a radio is keyed during the 0.5 second drop sequence it must waituntil the sequence is complete), and it offers absolute protection fromradios keying on top of each other on a working channel.

To make an Individual Call on the exemplary system, the calling radiouses a single inbound slot of the control channel to identify itself andto specify the radio being called. Both radios are referred, via anoutbound control channel message, to an available working channel wherethe confirmation signalling takes place. The unmute message to thecalled radio (at the completion of the high speed confirmationsignalling) also specifies the ID of the calling radio. The called radioautomatically stores the ID of the calling radio and, if the PTT switchof the called radio is depressed within 5 seconds of the last PTTrelease of the calling station, will automatically place an individualcall back to the original calling. This capability allows easyuser-convenient transmission trunking, and therefor better frequency(i.e., channel) efficiency, during individual calls. It also allows thecalling radio to contact a called radio and converse with no channelhang time even though the called radio is not previously programmed toinitiate a call to the calling radio.

The signalling in the exemplary embodiment is extremely efficient,minimizing the channel drop time and therefor increasing systemefficiency. It is unique in that, for example, it is high speedsignalling as opposed to low speed typically used on all other existingsystems. In addition, the signalling is designed specifically forminimizing message traffic in a distributed architecture site.

Without such novel channel drop signalling, as the channel startsdropping, a message would have to be sent from the working channel TC tothe control channel TC (via the site controller) stopping all updates onthe outbound control channel (i.e., those that are referring radios tothe working channel now being dropped). Once off the air, the channel TCwould have to send an additional message to the site controllerinforming it of such so it can reassign the dropped working channel whenappropriate. Besides additional messages within the central site slowingthe channel drop process, such prior techniques also incur additionalloading on the site controller. Another aspect of the problem is thatthe drop channel signalling that is transmitted on the working channelmust be of sufficient duration to guarantee that timing ambiguitiesdon't permit a radio to enter late onto the channel once it is down . .. or even worse to enter late onto the channel after the next call hasalready started to take place on that channel.

The exemplary embodiment uses unique drop channel signalling, a uniqueradio signalling detection algorithm and timing of when the channel TCsends the drop channel message to the site controller.

By making the drop channel signalling 9600 bps dotting, not only can thedrop channel signalling be detected and muted in radios prior to theradio operator hearing the signalling, but the detection algorithmplaces a light enough processor loading on the radios that they cansimultaneously look for the dotting and for confirmation signalling.

The following rules are followed by a working channel TC as it dross:

1) Transmit 100 msec of dotting.

2) Without interrupting the dotting, send a channel drop message to thesite controller.

3) Transmit an additional 200 msec of dotting . . . BUT . . . stop itand start sending a confirmation message should a channel assignmentmessage be received from the site controller.

The following rules are followed by the site controller when it receivesa drop channel message from a given channel TC:

1) Immediately inform the control channel TC so it can stop transmittingupdates to the working channel TC.

2) Consider the channel immediately available for reassignment.

The following rules are followed by a radio as it leaves a workingchannel:

1) For 1/2 second ignore all channel updates to the group and channel ofthe communique being left.

The following rules are followed by a radio as it arrives on a workingchannel:

1) Look for dotting (i.e., of sufficiently long duration to constitute adrop channel signal) and confirmation signalling simultaneously.

2) If drop-channel dotting is seen, leave the channel.

3) If confirmation is seen then leave the channel if the ID is notcorrect, otherwise lock onto the signalling and do not unmute until toldto do so.

4) If confirmation signalling stops, or no signalling is seen on thechannel, look for subaudible and unmute.

To understand the significance of the net effect of these procedures,consider two cases: (1) when the channel is not immediately reassignedand (2) when it is immediately reassigned.

It is only possible for the radio to enter late onto the drop channelsignalling 100 msec following the point when the drop channel messagewas sent to the site controller. So if the channel is not beingreassigned, a late entering radio will see the additional dotting beingtransmitted and will know to drop off the channel. On the other hand, ifthe system is loaded (e.g, call requests are queued in the sitecontroller) the channel immediately gets assigned to the first group inthe queue. A radio that attempts to enter late into the call justdropped will see the confirmation message with the group of the nextcall starting to take place and will know to drop off the channel.

The bottom line is that dropping a channel in a loaded system requiresonly 100 msec of signalling and only one message to the site controller.Radio's that happen to enter late into the call being dropped detectthat fact because of the radios ability to look for the drop channelsignalling and the confirmation signalling simultaneously.

For an increase in the radio price, the PST radio manufacturer mayprogram additional of these "features" into radios. The typical priorway to do this is to burn a unique PROM or an EEPROM at the factory. Onedisadvantage of this approach is the expense of uniquely programmingeach radio before it leaves the factory--and it is inefficient toupgrade a radio should a customer subsequently desire additionalfeatures.

However, the exemplary embodiment permits one to eliminate factoryprogramming costs. Since each radio is programmed in the field (e.g.,groups, systems, etc.) by the customer, features should be programmedinto the radio at that time. The problem is how to control theprogramming task sufficiently to make sure the customer only programspurchased features.

Every shipment of radios that goes to a customer will have a sheet ofpaper which lists a set of Programming Codes and Physical IDs (one pairfor each radio). Each Programming Code is an encryption of a "featureenable bitmap" and the Physical ID of the radio.

When a customer programs a radio he/she must do two things. First,he/she programs the radio. To do this, he/she selects the ProgrammingCode representative of the features he/she has purchased for the radioand enters it in the Radio Programmer. He/she then programs the radiousing the Radio Programmer while the Programming Code prevents him/herfrom programming disabled features. Second, the user enters the radioonto the system database via the system manager. The radio's Physical IDmust be specified in order to get the radio into the database.

Anytime the Radio Programmer writes data into a radio, it sets a "JustProgrammed" bit inside the radio's personality. Whenever a radio isturned on, it checks that bit. If set, the radio will use its PhysicalID to request a Logical ID from the site controller before allowing itsuser to communicate on the trunked system. The site controller will goto the system manager data base to determine the Logical ID to beassigned to the radio. Notice that if a customer tries using the sameProgramming ID for programming different radios he/she will end up withthe same Logical ID in each radio which means unique identificationcapability is lost. This is the same consequence suffered if a customercopied the PROM used in existing systems.

The result is that one has the same level of protection,--while avoidingthe need to program radios in the factory. Adding features to a radioinvolves issuing an updated Programming ID, ambiguities in programmingradios are eliminated (e.g., in existing systems a radio could beprogrammed to do something it was not enabled to do . . . so when acustomer programs it and it doesn't work he/she cannot tell whether theradio is erroneously programmed or the feature is disabled), and nospecial software is written in the mobiles . . . just the RadioProgrammer. This last benefit is nice since fixing a software bugrelative to feature enable/disable would involve changing code in just afew computers rather than for all radios in the field.

Detailed descriptions of the signalling protocols and formats involvedin many different types of call origination sequences are summarizedbelow:

I. Radio Origination, Logical ID Acquisition Sequence

A. The CC transmits a continuous stream of control messages which allinactive mobiles receive. The messages are sent two messages to a30-msec frame in the following frame format:

Dotting=32 bits

Barker=16 bits (e.g., 11 bits Barker code plus 5 bits dotting preamble)

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. When mobile power is turned on, it receives a site ID message fromthe Control Channel (CC)in the following format:

MT-A=2 bits (e.g.,11)

MT-B=3 bits (e.g.,111)

MT-C=4 bits (e.g.,1110)

Delay=2 bits

Channel=5 bits

Priority=3 bits

Homesite=1 bit

Failsoft=2 bits

Site ID=6 bits

BCH code=12 bits

Delay specifies the maximum number of control channel slots before acontrol channel responds to an inbound transmission. Channel specifiesthe channel number for the active control channel. Priority prohibitsmobiles with lower priority from transmitting on the inbound controlchannel. Home site bit specifies whether the site ID is the home (=0) oradjacent (=1) ID.

C. If desired, and if priority allows, mobile optionally transmits alogin request on the control channel in synchronism with the receivedcontrol channel messages. The frame form is as follows:

Dotting=152 bits

Barker Code (repeated three times)=48 bits (including filler)

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The login message is coded as follows:

MT-A=2 bits

MT-B=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

If the mobile has no logical ID, it will transmit the logical ID requestmessage The logical ID request message is coded as follows:

MT-A=2 bits

MT-B=3 bits

MT-C=3 bits

Physical ID=20 bits

BCH Code=12 bits

D. The control channel responds with a logical ID assignment message.

II. Radio Call Sequence--Radio Origination, Group Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a group call transmits a groupchannel assignment request on the control channel in synchronism withthe received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The group call request message is coded as follows:

MT-A=2 bits

Type Communications (e.g., voice, data, interconnect or voice privacy)=2bits

Not used=1 bit

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-messagepair. Coding is as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channeland receive a confirmation message. Slotted working channel messages aretransmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmutemessages.

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Filler=1 bit

Logical ID=12 bits

BCM Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels receive a subaudible channelassignment message.

H. Control channel transmits channel update message for late entrymobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. Allnon-slotted message formats are:

Dotting=384 bits

Data Block #3=128 bits

Data Block #2=128 bits

Data Block #1=128 bits

Date Block #0=128 bits

Data Block #3, #2, #1 and #0 are identical--except for a two bit blockcount--(each block is repeated four times) and each has the followingformat:

    ______________________________________              Dotting = 16 bits              Barker Code = 16 bits              Byte 1 = 8 bits              Byte 1 (inverted) = 8 bits              Byte 1 = 8 bits              Byte 2 = 8 bits              Byte 2 (inverted) = 8 bits              .              .              .              Byte 3 = 8 bits              Byte 4 = 8 bits              Byte 4 (inverted) = 8 bits              Byte 4 = 8 bits    ______________________________________

The unkey message is coded as follows:

MT Code=4 bits

Unused=2 bits

Block Count=2 bits

Logical ID=12 bits

BCH Code=12 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop allmobiles from the channel.

III. Radio Call Sequence-Radio Origination, Individual Call

A. Control transmits a continuous stream of control messages which allinactive mobiles receive.

The messages are sent two messages to a 30-msec. frame that has thefollowing format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an individual call transmits anassignment request on the control channel in synchronism with thereceived control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The individual call request message is coded as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

Logical ID (called)=12 bits

Logical ID (caller)=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-messagepair. Coding is as follows:

MT-A Code=2 bits

Type Communications (e.g., voice)=2 bits

1/2 Logical ID=6 MSBs or 6 LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Both calling (last logical ID) and called mobile switch to assignedworking channel and receive a confirmation message. Slotted workingchannel messages are transmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmutemessages:

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

Unused bit=1 bit

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels do not receive a subaudiblechannel assignment message.

H. Control channel transmits channel update message for late entrymobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. Allnon-slotted message formats are:

Dotting=384 bits

Data #3=128 bits

Date #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #2, #1, and #0 are identical (repeated four times) and each hasthe following format:

    ______________________________________              Dotting = 16 bits              Barker = 16 bits              Byte 1 = 8 bits              Byte 1 (inverted) = 8 bits              Byte 1 = 8 bits              Byte 2 = 8 bits              Byte 2 (inverted) = 8 bits              .              .              .              Byte 3 = 8 bits              Byte 4 = 8 bits              Byte 4 (inverted) = 8 bits              Byte 4 = 8 bits    ______________________________________

The unkey message is coded as follows:

MT Code=4 bits

Unused=2 bits

Subcount=2 bits

Logical ID=12 bits

BCH Code=12 bits

IV. Radio Call Sequence-Radio Origination, Emergency Group Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=11 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an emergency group call transmits anassignment request on the control channel in synchronism with thereceived control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The emergency group call request message is coded as follows:

MT-A Code=2 bits

Type communications=2 bits

Status/C=1 bit

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with two channel-assignment messagesthat are coded as follows:

MT-A Code=2 bits

Type communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channeland receive a confirmation message. Slotted working channel messages aretransmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The emergency group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible count=2 bits

Message/Transmission Trunking=1 bit

Group/Logical ID 1 bit

Group ID=12 bits

BCH=12 bits

E. Originating mobile receives confirmation message and transmits384-bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmutemessages.

MT Code=4 bits

Subaudible Count=2 bits

Message/Transmission Trunking=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels receive subaudible channelassignment message.

H. Control channel transmits channel update message for late entrymobiles.

I. Transmitting mobile unkeys and sends two non-slotted unkey messages.All non-slotted message formats are:

Dotting=384 bits

Data #3=128 bits

Data #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #1, #1 and #0 are identical (repeated four times) and each hasthe following format:

    ______________________________________              Dotting = 16 bits              Barker = 16 bits              Byte 1 = 8 bits              Byte 1 (inverted) = 8 bits              Byte 1 = 8 bits              Byte 2 = 8 bits              Byte 2 (inverted) = 8 bits              .              .              .              Byte 3 = 8 bits              Byte 4 = 8 bits              Byte 4 (inverted) = 8 bits              Byte 4 = 8 bits    ______________________________________

The unkey message is coded as follows:

MT=Code=4 bits

Subaudible Count=2 bits

Logical ID=12 bits

BCH Code=12 bits

V. Radio Call Sequence-Radio Origination, Status Call

A. Control channel transmits a continuous steam of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a status call transmits a statusrequest on the control channel in synchronism with the received controlchannel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The status request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

3 bits (unused)=000

Auto Response=1 bit (e.g., yes)

4 bits (unused)=0000

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a status page message that is codedas follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=4 bits

2 bits (unused)=00

Auto response=1 bit (e.g.,yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

D. Called mobile transmits a control channel status message that iscoded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

3 bits (unused)=000

Auto Response=1 bit (e.g., yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

E. Control channel responds with a status acknowledge message that iscoded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=4 bits

2 bits (unused)=00

Auto Response=1 bit (e.g., yes)

Status=4 bits

Logical ID=12 bits

BCH Code=12 bits

Originating mobile receives status message.

VI. Radio Call Sequence-Radio Origination, Special Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a special call transmits a specialcall request on the control channel in synchronism with the receivedcontrol channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The special call request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

MT-C Code=3 bits

2 bits (unused)=00

Type Communications Code=2 bits (e.g., interconnect)

1 bit (unused)=0

Priority Code=3 bits

Logical ID=12 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-messagepair. Coding is as follows:

MT-A Code=2 bits

Type Communication Code=2 bits (e.g.,intent)

1/2 Logical ID=6 MSBs or LSBs

Group/Logical=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Mobile switches to the assigned working channel and receives aconfirmation message. Slotted working channel messages are transmittedusing the following frame.

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call confirmation message is coded as follows:

MT Code=4 bits

Subcount=2 bits

Hang/time/Trunked=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits amulti-block special call message. The message frame (shown below) canhave from 1 to 16 blocks.

    ______________________________________             Dotting = 384 bits             Data #3 Block #1 = 128 bits             Data #2 Block #1 = 128 bits             Data #1 Block #1 = 128 bits             Data #0 Block #1 = 128 bits             Data #3 Block #2 = 96 bits             Data #2 Block #2 = 96 bits             .             .             .    ______________________________________

Data #3, #2, #1, #0 are identical in each block (repeated four times).Coding for Block #1 Data is:

    ______________________________________              Dotting = 16 bits              Barker = 16 bits              Byte 1 = 8 bits              Byte 1 = (inverted) = 8 bits              Byte 1 = 8 bits              Byte 2 = 8 bits              Byte 2 (inverted) = 8 bits              .              .              .              Byte 3 = 8 bits              Byte 4 = 8 bits              Byte 4 (inverted) = 8 bits              Byte 4 = 8 bits    ______________________________________

Data in blocks after block #1 do not have dotting or Barker code. Iftelephone interconnect is required, block #1 data is coded as follows:

Group Count=4 bits

Individual Count=4 bits

Phone Digit Count=4 bits

Phone Digit #1=4 bits MSD

Phone digit #2=4 bits

BCH Code=12 bits

If no interconnect is required, block #1 is coded as

Group Count=4 bits

Individual Count=4 bits

Group/Logical ID=12 bits

BCH Code=12 bits

Subsequent blocks are coded with either one group ID, one logical ID, orfive telephone digits as required to satisfy the block #1 counts. Thetelephone digits first, then the logical ID's, then the group ID's.Digit coding is one digit per nibble (null=1010). ID coding is

8 Bite=10101010 (8 bits)

Group/Logical ID=12 bits

BCH Code=12 bits

F. Working channel transmits a slotted working channel special callreceive bitmap message. Slotted working channel messages are transmittedusing the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call receive bit map is coded as follows (use of similaracknowledgment bit maps is the subject of related commonly assignedcopending application No. 07/056,923 filed Jun. 3, 1987 (GE docket45-MR-496)):

    ______________________________________           MT Code = 4 bits           Block #1 bit = 1 bit (e.g., OK)           Block #2 bit = 1 bit (e.g., OK)           Block #3 bit = 1 bit (e.g., O = repeat)           Block #4 bit = 1 bit           .           .           .           Block #16 bit = 1           BCH Code = 12 bits    ______________________________________

G. Originating mobile receives bitmap message and transmits amulti-block special call message. The message frame (shown below) canhave from 1 to 16 blocks.

    ______________________________________             Dotting = 384 bits             Data #3 Block #1 = 128 bits             Data #2 Block #1 = 128 bits             Data #1 Block #1 = 128 bits             Data #0 Block #1 = 126 bits             Data #3 Block #2 = 96 bits             Data #2 Block #2 = 96 bits             .             .             .    ______________________________________

Data #3, #2, #1, #0 are identical in each block (repeated four times).Coding for block #1 is

    ______________________________________              Dotting = 16 bits              Barker = 16 bits              Byte 1 = 8 bits              Byte 1 (inverted) = 8 bits              Byte 1 = 8 bits              Byte 2 = 8 bits              Byte 2 (inverted) = 8 bits              .              .              .              Byte 3 = 8 bits              Byte 4 - 8 bits              Byte 4 (inverted) = 8 bits              Byte 4 = 8 bits    ______________________________________

Data in blocks after block #1 do not have dotting or Barker code.

Only blocks that have a "O" in their bitmap bit are transmitted. Forexample in step F, block #3 in step E would be the first blockretransmitted. If no bit map is received within 100 msec. after steps Eor G, all blocks are retransmitted.

Steps F and G are repeated until all blocks are received correctly(BITMAP=1's).

H. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

I. The control channel sends from 0 to 16 channel-assignment two-messagepairs as required for the special call. Coding for each message is asfollows:

MT-A Code=2 bits

Type Communications Code=2 bits

1/2 Logical ID=6 MSBs or 6 LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical=12 bits

BCH Code=12 bits

J. All called mobiles go to the assigned working channel and unmute(same as late entry). From this point the working channel messages arethe same as a group or individual call.

VII. Radio Call Sequence-Radio Origination, Dynamic-Regroup Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has thne following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a dynamic-regroup call transmits arequest on the control channel in synchronism with the received controlchannel messages. The frame format is as follows:

Dotting=152 bits

Barker=(repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The dynamic-regroup request message is coded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID-12 bits

BCH Code=12 bits

C. The control channel responds with a dynamic-regroup message that iscoded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

D. Mobile acknowledges the dynamic regroup with a login message that iscoded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

E. Mobile may request cancellation of the dynamic-regroup with a messagecoded as follows:

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

BCH Code=12 bits

F. Control channel responds with a cancel dynamic-regroup message thatis coded

MT-A Code=2 bits

MT-B Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

G. Mobile acknowledges the dynamic-regroup cancellation with a loginmessage that is coded

MT-A Code=2 bits

LMT-3 Code=3 bits

Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

VIII. Radio Call Sequence-Console Origination, Group Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate a group call transmits a group callmessage to the downlink. The group call message is coded as follows:

MID=1 byte (#0)=8 bits

# bytes=1 byte (#1)=8 bits

Source destination=bytes #2 and #3=16 bits

Not Used=4 bits

MT-A Code=2 bits

Type communication=2 bits

Group ID=12 bits

Logical ID=12 bits

Parity=1 byte (#8)=8 bits

C. The control channel responds with a channel-assigrment two-messagepair. Coding is as follows:

MT-A Code=2 bits

Type Communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Group ID=12 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channeland receive a confirmation message. Slotted working channel messages aretransmitted using the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Group ID=12 bits

BCH Code=12 bits

E. Originating console receives channel-assignment from the downlink andswitches audio to the specified channel. Console message is coded asfollows:

MID=1 Byte (#0)=8 bits

# Bytes=1 Byte (#1)=8 bits

S/D=Bytes #2, #3=16 bits

Not Used=4 bits

MT Code=2 bits

Type communications=2 bits

Logical ID-12 bits

Not Used=2 bits

GR/L ID=1 bit

Channel=5 bits

Group ID=12 bits

Parity=1 Byte (#9)=8 bits

F. Working channel transmits two unit-keyed/unmute messages:

MT Code=4 bits

Subaudible Count=2 bits

Hang-time/trunked=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobiles receive unmute message and unmute audio.

G. Active mobiles on other working channels receive a subaudible channelassignment message.

H. Control channel transmits channel update message for late entrymobiles.

I. Console sends unkey message that is coded as follows:

MID=1 Byte (#0)=8 bits

# Bytes=1 byte=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not used=4 bits

MT Code=4 bits

Not Used=4 bits

Logical ID=12 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop allmobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

Bytes=Byte #1=8 bits

Source Designation Bytes=#2 & #3=16 bits

MT-A/B/C=9 bites

Drop Cr=1 bit

Not Used=1 bit

Channel=5 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

IX. Radio Call Sequence-Console Origination, Individual Call

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are sent two messagesto a 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate an individual call transmits anindividual call message to the downlink. The individual call message iscoded as follows:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits

Type Communication=2 bits

Logical ID=12 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

C. The control channel responds with a channel-assignment two-messagepair. Coding is as follows:

MT-A Code=2 bits

Type Communications=2 bits

1/2 Logical ID=6 MSBs or LSBs

Group/Logical ID=1 bit

Channel=5 bits

Logical ID=12 bits

BCH Code=12 bits

D. Called mobile switches to assigned working channel and receives aconfirmation message. Slotted working channel messages are transmittedusing the following frame:

Dotting=32 bits

Barker=16 bits

Message=32 bits

Message inverted=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Subaudible Count=2 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Logical ID=12 bits

BCH Code=12 bits

E. Originating console receives channel-assignment message from thedownlink and switches audio to the specified channel. Console message iscoded as follows:

MID=Byte #0 (8 bits)

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=2 bits

Type Communication Code=2 bits

Logical ID=12 bits

Not used=2 bits

GR/L ID=1 bit

Channel=5 bits

Logical ID=12 bits

Parity=Byte #9=8 bits

F. Working channel transmits two unit-keyed/urinute messages.

MT Code=4 bits

Subaudible Count=2 bits

Hang-Time/Trunked=1 bit

1 bit (unused)=0

Logical ID=12 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels do not receive a subaudiblechannel assignment message.

H. Control channel transmits channel update message for late entrymobiles.

I. Console sends unkey message that is coded as follows:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT Code=4 bites

Not Used=4 bits

Logical ID-12 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop allmobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 and #3=16 bits

MT-A/B/C=9 bites

Drop Ch=1 bit

Not Used=1 bit

Channel=5 bites

Logical ID=12 bits

Parity=Byte #8=8 bits

X. Radio Call Sequence Console Origination, Activate Patch

A. Control channel transmits a continuous stream of control messageswhich all inactive mobiles receive. The messages are set two messages toa 30-msec. frame that has the following format:

Dotting=32 bits

Barker=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to establish a patch transmits a patch IDassignment message to the downlink. The patch ID assignment message isvariable length depending upon the group and individual ID counts:

MID=29=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3=16 bits

Not Used=4 bits

Group Count=4 bits

Individual Count=4 bits

Logical ID=12 bits

Not Used=12 bits

Logical ID=12 bits

Not Used=12 bits

Logical ID=12 bits

Not Used=13 bits

Group ID=11 bits

Not Used-13 bits

Group ID=11 bits

Not used=13 bits

Patch ID=11 bits

Parity=8 bits

C. Console receives an acknowledgement of the patch request from thesite controller using a special group ID code (1000 0000 0000):

MID=12=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3=16 bits

Not Used=4 bits

MT-A Code=11=2 bits

MT-B Code=100=3 bits

Patch ID=11 bits

Group ID=11 bits

Parity=Byte #8=8 bits

D. When console wishes to activate the patch it transmits a patchactivate message to the downlink.

MID=27=Byte #0=8 bits

# Bytes=Byte #1=05=8 bits

Source/Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=4 bits (1110)

Not Used=5 bits

Patch ID=11 bits

Parity=Byte #7=8 bits

E. The control channel responds with alias ID assignment messages. Groupassignment messages are coded:

MT-A Code=2 bits (11)

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Not Used=1 bit

Group ID=11 bits

BCH Code=12 bits

Group alias ID messages are repeated in the control channel backgroundmode and not acknowledged by the mobiles.

Individual alias ID assignment messages are coded as follows:

MT-A Code=2 bits (11)

MT-B Code=3 bits (101)

Alias Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

F. All mobiles receive assignment messages but only individual callmobiles acknowledge message.

MT-A Code=2 bits (11)

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Logical ID=12 bits

BCH Code=12 bits

G. Console receives patch assignment/activate messages to confirm patch.One message for each assignment:

MID=12=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits (11)

MT-B Code=3 bits (100)

Patch ID=11 bits

Group ID=12 bits

Parity=Byte #8=8 bits

MID=13=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=2 bits (11)

MT-B Code=3 bits (101)

Patch ID=11 bits

Logical ID=12 bits

Parity=Byte #8=8 bits

H. Console originates a patch call by transmitting a group call messageusing the patch ID:

MID=24=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT Code=2 bits (00)

Type Communication=2 bits (00)

Patch ID=12 bits

Logical ID=12 bits

Parity=Byte #8

I. Control channel transmits group call message. Subsequent steps aresame as console originated group call.

While only one exemplary embodiment of this invention has been describedin detail, those skilled in the art will recognize that many variationsand modifications may be made in this embodiment while still retainingmany of its novel features and advantages. Accordingly, all suchmodifications and variations are intended to be included within thescope of the appended claims.

What is claimed is:
 1. In a trunked radio repeater system for achievingRF communication between at least a first radio unit and a second radiounit over an RF working channel temporarily assigned to carrycommunications between said at least first and second radio units, amethod of dynamically specifying transmission trunked or message trunkedusage on a call-by-call basis for said temporarily assigned workingchannel, said method comprising the steps of:carrying digitally encodedworking channel request and assignment signals associated with a callover a full duplex digital RF control channel; monitoring said controlchannel at said first and second unit, and switching to a workingchannel temporarily assigned to said call based on said digitallyencoded working channel assignment signals carried over said controlchannel; and dynamically specifying transmission trunked or messagetrunked usage for said call by specifying trunked call type in at leastone digital information field included within digitally encoded signalsassociated with said call that are carried over at least one of saidcontrol channel or said temporarily assigned working channel.
 2. Amethod as in claim 1 further comprising the steps of:storing, in one ofsaid first and second radio unit, the identity of the other unit inresponse to receipt of said channel assignment signals; and in responseto a specified call type indicating transmission trunked usage,automatically generating, with said one radio unit, channel requestsignals including said stored identity of said other radio unit so as toautomatically generate channel request signals on the control channel tocall back the other radio unit over a further temporarily assignedworking channel.
 3. The method as in claim 2, wherein the step ofautomatically calling back is performed with no channel hang time andincludes sending at least some 9600 bps signaling over the controlchannel.
 4. The method as in claim 2, wherein said first and secondunits each include push to talk (PTT) switches, and the step ofautomatically calling back is performed if the push to talk (PTT) switchof the one unit is depressed within a predetermined time after releaseof the push to talk (PTT) switch of the other unit.
 5. A method as inclaim 1 wherein:said dynamically specifying step includes transmittingover the air with at least one radio repeater substantially at 9600 bitsper second on at least one of said control channel or said temporarilyassigned working channel, a digital transmission/message trunked statussignal identifying whether the usage of the temporarily assigned workingchannel is to be transmission or message trunked; and wherein saidmethod further includes controlling call termination timing with atleast one of said first and second radio units based at least in part onsaid trunked status signal.
 6. A method as in claim 1 wherein saidcarrying step comprises carrying said digitally encoded working channelrequest and assignment signals over said digital control channel at arate of substantially 9600 bps and using a substantial portion of the9600 bps signalling rate capacity to provide enhanced signallingreliability.
 7. A method as in claim 1 further including carrying voicesignals between said first and second units over said temporarilyassigned working channel.
 8. A method as in claim 1 further includingcarrying digital signals substantially at 9600 bps over said temporarilyassigned working channel, and using a substantial portion of the 9600bps signalling rate capacity to provide enhanced signalling reliability.9. A method as in claim 1 further including specifying the identity ofsaid first radio unit and the identity of said second radio unit in atleast some of said digitally encoded signals, and wherein saiddynamically specifying step comprises including, within said digitallyencoded signals, at least one digital message/transmission trunking bitthat determines, independently of said first radio unit identity andsaid second radio unit identity, whether said first and second radiounits are to engage in a transmission trunked call or in a messagetrunked call.
 10. A method as in claim 1 wherein said dynamicallyspecifying step comprises including, within digitally encoded signalstransmitted over said temporarily assigned working channel, at least onedigital message/transmission trunking bit that determines whether saidfirst and second radio units are to engage in a transmission trunkedcall or in a message trunked call.
 11. A method as in claim 1 furtherincluding automatically inhibiting, in a transmission trunked usage, oneof said first and second radio units from transmitting on saidtemporarily assigned working channel.
 12. A method as in claim 1 furtherincluding deallocating said temporarily assigned working channel:(a) ina transmission trunked usage, in response to unkeying of said firstradio unit, and (b) in a message trunked usage, a predetermined timefollowing unkeying of either said first radio unit or second radio unit,unless the other radio unit keys within said predetermined time.
 13. Amethod as in claim 1 further including, in a transmission trunked usage,substantially immediately reverting said first and second radio units tosaid control channel upon unkeying, and freeing said temporarilyassigned working channel for reassignment.
 14. A method as in claim 1further including, in transmission trunked usage, beginning to transmitchannel drop signaling on said temporarily assigned working channel inresponse to receipt of an unkey message from a transmitting one of saidfirst and second radio units without waiting for said transmitting radiounit to switch to a receive operation for reliably detecting saidchannel drop signaling.
 15. Apparatus for achieving reliable and promptcommunication within a trunked radio repeater system having a digitalcontrol channel and plural working channels, which working channels areassigned for temporary use of individual radio units specified bydigital control signals on the control channel, said apparatuscomprising:at least one repeater transceiver that transmits andreceives, over said control channel and over at least one temporarilyassigned working channel, digital signals associated with a particularcommunique; and at least one mobile or portable radio transceiver thatreceives and detects as part of said digital signals associated withsaid particular communique, on at least one of said control channel oran assigned working channel, a call type signal identifying whether theusage of the working channel for said particular communique is to betransmission or message trunked.
 16. A method of operating a digitallytrunked radio communication system including at least one radio unitcapable of operating in either of a transmission trunked mode and amessage trunked mode, said method comprising the steps of:(a) sending,over the air as part of digital signalling associated with a particularcommunique involving said radio unit:(1) a first digital codeidentifying said radio unit; (2) a second digital code identifying aworking channel, and (3) a third digital code specifying transmission ormessage trunking usage of said identified working channel for saidparticular communique; (b) receiving said first, second and thirddigital codes with said radio unit; and (c) in response to said digitalcodes received by said receiving step (b), temporarily operating saidradio unit identified by said first digital code, for that particularcommunique, on said working channel identified by said second code insaid trunking mode identified by said third digital code.
 17. A methodas in claim 16 further comprising the steps of:leaving said workingchannel and returning to said digital control channel substantiallyimmediately upon completion of a transmission, when said third digitalcode has a first predetermined value indicating transmission trunking;and waiting on said working channel for a predetermined delay time aftercompletion of a transmission to receive a possible response from saidtransmission over said same temporarily assigned working channel, whensaid third digital code has a second predetermined value indicatingmessage trunking.